Carrier sense multiple access with collision detection

In computer networking, a carrier sense multiple access with collision detection (CSMA/CD) is a network control protocol in which (a) a carrier sensing scheme is used and (b) a transmitting data station that detects another signal while transmitting a frame, stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to send that frame again.

Multiple Access means that multiple (really: more than two) stations can share (or access) the network at a time. The alternative is often called point-to-point and has only two stations that access the network segment.

Carrier Sense means that a station that has data to transmit listens for a carrier before transmitting. This reduces the likelihood that two stations will transmit at the same time. The carrier is a communications theory term that describes how the signal is encoded. The station with data to transmit usually listens for about two (2) cable-delay times.

Collision Detect means that a transmitting station listens to the media, comparing the data it is transmitting with the data being received (seen) on the media. If they differ, there must be another station transmitting different data at the same time - a collision. The back-off algorithm is engaged at this point in case this is a multiple collision.

A CSMA protocol can run without CD - the receiving station will detect an error in the received data, usually by calculating a check-sum (e.g., parity or CRC) - however, this is usually less efficient and has a lower theoretical bandwidth.

Ethernet is the classic CSMA/CD protocol.

Very partly derived from Federal Standard 1037C